The continuing integration of portable electronics into daily life and the benefits to society thereof have been dependent on increasing both the performance and portability of electronic systems. These requirements conflict because the underlying integrated circuits (ICs) that enable electronic devices require greater amounts of power to provide a greater degree of functionality and accuracy. Digital circuits consume more power with greater functionality because they require more devices that burn additional current while switching. Analog circuits require more power to perform with greater accuracy because larger voltage and current signals are less susceptible to noise and manufacturing asymmetries. Electronic circuits in general consume more power when required to perform faster. The power and performance tradeoff that is inherent in ICs creates an acute design challenge in the field of portable electronics.
Voltage regulators are a particular kind of circuit that is necessary for the functionality of portable electronics. Portable electronic devices are powered by batteries with voltages that differ from the voltages required by the device's ICs. For example, a battery cell voltage for a mobile phone could be 3.6V while the microprocessor IC of the mobile phone might require a 1.8V supply voltage. A voltage regulator is used to take in the battery voltage of the device as an input voltage and output the supply voltage to the IC. The IC is often referred to as the load of the regulator. Voltage regulators are also called DC to DC converters owing to the fact that they convert one DC voltage to another.
Ideal voltage regulators supply the same voltage regardless of the current drawn by the load IC. In addition, an ideal voltage regulator does not consume any power and provides a clean output voltage to the IC. The two most common types of voltage regulators are switching regulators and linear regulators. Linear regulators are also referred to as series regulators or low-dropout (LDO) regulators in reference to their most common configuration. Switching regulators are sometimes referred to as pulse-width modulated (PWM) regulators or buck regulators in reference to a common switching regulator architecture. Switching regulators introduce variation to the output voltage but are generally very power efficient. Linear regulators provide a clean output voltage but can be very inefficient under certain operating conditions.
The operation of a linear regulator can be explained with reference to FIG. 1. In FIG. 1, the voltage VIN is the input voltage and VOUT is the output voltage. In this particular example, the regulator is a step-down voltage regulator as VOUT has a lower potential than VIN. Capacitor 101 is placed across load 102 to provide stability to the system. Load 102 will draw varying amounts of current depending upon its state. The feedback network formed by resistors 103 and 104 and feedback circuit 105 will alter the voltage at VG to keep VOUT at the target voltage. If load 102 suddenly drew more current, the feedback network would lower the voltage at VG and more current would flow through pass transistor 100 to compensate for the additional load current. The same process would occur in reverse if load 102 suddenly drew less current.
The linear voltage regulator does not introduce any additional variation to the regulated voltage, but it can consume a significant amount of power. The current through pass transistor 100 is controlled by a continuous feedback signal so no additional variation is added to the voltage on VOUT by the linear voltage regulator. However, since the voltage drop from VIN to VOUT is placed across transistor 100, energy is dissipated as heat in proportion to the voltage across the transistor. Neglecting the power consumption associated with the feedback loop, the efficiency equation for the linear voltage regulator in FIG. 1 is:η=VOUT/VIN Using the numbers associated with the previous example of the mobile phone battery and microprocessor, the best possible efficiency a linear regulator could achieve would be 50%.
The operation of a switching regulator can be explained with reference to the circuit in FIG. 2 and the waveforms in FIG. 3. In FIG. 2 the voltage VIN is the input voltage and the voltage VOUT is the output voltage. Feedback circuit 204 receives a voltage related to VOUT from the resistor network comprised of resistors 203 and 207. Based on the received signal, feedback circuit 204 sends alternating signal to nodes VG1 and VG2 such that either transistor 200 or 202 is on and the other is off. The voltages at VG1 can be seen on axis 300 and the voltage at VG2 can be seen on axis 302. Current I1, which flows through transistor 200, is shown on axis 301 and current I2, which flows through transistor 202, is shown on axis 303. The voltage at node VA is shown on axis 304 and the voltage at node VOUT is shown on axis 305.
A switching voltage regulator is continuously altering between two phases as the switching signal turns on either transistor 200 or transistor 202. The first phase begins when VG1 and VG2 are set to low. At the time just before VG1 is set to a low value and transistor 200 is switched on, the minimum current for a given load condition is flowing through inductor 201. Once the voltage at VG1 is set to a low value inductor 201 will continue to pull the same minimum current from VIN through transistor 200. The current will flow through inductor 201 to capacitor 205 and load 206. The current through inductor 201 will build slightly during this phase as the voltage potential across the inductor will be in the same direction as the current flow. This increase in current will charge capacitor 205 and will then begin to supply more current than load 206 requires. At this point, feedback circuit 204 will switch the regulator into its second phase.
In the second phase, both VG1 and VG2 will be set to a high value such that transistor 200 is off and transistor 202 is on. When the regulator switches, the current through inductor 201 will be at its maximum for a given load condition. Inductor 201 will continue to pull the same maximum current because of the stored electromotive force (EMF) in the inductor. This force will push node VA down to below ground potential and charge will flow from ground through transistor 202 to node VA. Since the voltage potential across the inductor will be in the opposite direction of the current flow, the EMF in the inductor will dissipate during this phase. The current through inductor 201 will decrease until the regulator is supplying slightly less current than load 206 requires. At such time the system will switch back to the first phase.
The switching regulator introduces a certain degree of variation in the regulated voltage but it consumes relatively little power. The reason switching regulators are so efficient can be explained with reference again to FIG. 2 and FIG. 3. As shown on axis 304, the voltage drop across transistors 200 and 202 is very low while either transistor is conducting. The main voltage drop from VIN to VOUT is applied across the inductor where it is stored by generating EMF as opposed to being applied across a transistor and dissipated as heat. Neglecting the power consumption associated with the feedback circuitry, the efficiency of a switched voltage regulator can be around 95%. However, as mentioned previously, the switching action of the transistors produces variations in the output voltage. An example of this variation is shown on axis 305.
If the power consumption of the regulator's control and feedback circuitry is taken into account, the switching regulator's efficiency advantage is diminished. A switching regulators feedback circuitry must charge and discharge the gates of the switching transistors continuously and uses a power hungry feedback circuit whereas the power consumption in a linear regulator's feedback circuitry consists mainly of a single amplifier. In general, the linear regulator's feedback circuitry does not consume as much power as that of a switching regulator. Since the control circuitry of both regulators is not directly dependent on the output current, when the load current of a regulator decreases a switching regulator loses its efficiency advantage.
The tradeoffs associated with linear and switching regulators have led to many approaches that synergize the two designs. For example, U.S. Pat. No. 5,309,082 to Payne uses a linear regulator and switching regulator in cascade to take advantage of the benefits of the two regulator types while masking the low efficiency of a stand-alone linear regulator. U.S. Pat. No. 7,084,612 to Zinn uses a switching regulator and linear regulator in series to provide an advantageous mix of the two regulator's characteristics. In addition, there is a large body of patents that focus on combining aspects of linear and switching regulators by operating the two types of regulators in parallel or operating them individually based on the current drawn by the load.
A linear and switching regulator can be operated in parallel to form a single regulator in order to take advantage of the speed superiority of a linear regulator and the power efficiency of a switching regulator. In U.S. Pat. No. 5,258,701 to Pizzi and U.S. Pat. No. 6,636,023 to Amin, a linear regulator and switching regulator are placed in parallel but the linear regulator only turns on when the output voltage drops below a certain threshold that is lower than the target regulated voltage. In such a circuit the linear regulator only turns on when the regulated voltage has varied wildly and the speed of the linear regulator is required to correct the variation quickly. When the load is stable, the linear regulator will turn off and the highly efficient switching regulator will regulate the output voltage so the overall efficiency of the combined regulator is maximized. U.S. Pat. No. 6,661,211 to Currelly is similar in that the linear regulator is only active during circuit start-up when the linear regulators speed is required.
A single regulator comprised of a parallel linear and voltage regulator can also mask the dramatic decrease in efficiency caused by an increase in the voltage drop from the input to output voltage. For example, U.S. Pat. No. 6,150,798 to Ferry describes a voltage regulator system containing both a switching and linear voltage regulator and a control circuit that switches between the two regulators based on the differential between the input and output voltages. For small differentials the linear regulator is applied to regulate the voltage and at large differentials the linear regulator is turned off and a switching regulator is applied. This is advantageous from a power optimization perspective because the efficiency of the linear regulator is directly related to this differential. The linear voltage regulator and its clean output voltage can be applied until doing so would waste too much power. U.S. Pat. No. 7,190,150 to Chen applies a similar principle for regulating the voltage applied to power amplifiers.
Another family of circuits that utilize linear and switching regulators in parallel addresses the decreased efficiency of switching regulators at low load currents. U.S. Pat. No. 6,597,158 to Umeda and U.S. Pat. No. 5,773,966 to Steigerwald describe a voltage regulator that operates in switch mode for high load currents and changes to a linear regulator when a measurement and control circuit detects a decrease in the load current past a certain level. U.S. Pat. No. 6,424,128 to Hiraki et al. uses a similar method but allows for the use of multiple linear regulators. U.S. Pat. No. 7,064,531 to Zinn builds on the general idea of this family of circuits by using a linear regulator with a relatively small pass element compared to the switching regulator's pass transistors. As such, the low load current efficiency of the linear regulator is improved to an even greater degree because the feedback and control circuitry needs to supply less charge to the lower capacitance associated with the smaller device. There are other circuits known in the art that transition between the low current linear regulator states to high current switching regulator states automatically based on the output current. Instead of monitoring the output current with control circuitry, these circuits are designed such that the linear regulator turns off automatically as the load current increases. U.S. Pat. No. 4,502,152 to Sinclair and U.S. Pat. No. 5,034,676 to Kinzalow are examples of this kind of circuit.
Circuits are known in the art that apply either a linear or switching regulator configuration based on the operating mode of the load they are regulating. For example, U.S. Pat. No. 6,815,935 to Fujii takes advantage of the lower output current power consumption superiority of a linear regulator by applying a linear voltage regulator to the load when its regulated device is in a standby mode and applying a switching voltage regulator when the load is in its basic operational mode. This approach is similar to that implemented by Umeda but it selects between the two regulators based on an input signal that controls the operating state of the load instead of one that is generated by measuring the operating state of the load.
Another design that utilizes switching and linear voltage regulators in parallel and applies them based on the operating condition of the load is described in U.S. Pat. No. 5,502,369 to Niwayama. The patent describes a voltage regulator that supplies a voltage to a device based on the operating mode of the device. The voltage regulator will apply either a switching or linear regulator output to the device depending upon the regulated device's operating mode. For example, when a CD player is operating normally a linear regulator is applied to regulate the supply. Contrarily, when the CD player is ejecting a disc and requires a relatively large amount of power, a switching regulator output is supplied to the CD player. As such, the circuit applies the proper type of regulator based on the needs of the load device. When the device is in a mode in which a large amount of power is required but supply stability and accuracy are not important, a switching regulator is applied. When the device is in a mode that uses a smaller amount of power but accuracy is very important, a linear regulator is applied.